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:: Home > Watersheds > Vegetation and Topsoil > Filtration

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Small streamWhen precipitation falls in a watershed in the form of rain or snow, much of the water filters into the ground.  This process of filtration helps purify the water that ends up as ground water, which may later end up being the water coming out of a drinking well, irrigation pump, or natural spring.

Vegetation and topsoil are keys to maintaining a healthy filtration system that benefits people, animals, fish, and plants.  In streams and rivers, large woody debris and rocks help to further filter and purify the water by slowing water flow and allowing sediment and suspended pollutants to settle to the stream bottom. In addition, these structures within a stream help to reinforce the structure of a stream, which helps reduce the impact of flooding.  They also serve fish and other aquatic species as protection from predators, swift currents, and damaging rays of sunlight, while also providing sources for food and foraging. 

Vegetation & woody debrisVegetation and woody debris help to filter sediments by trapping them behind hundred or thousands of tiny dams or strainers. Vegetation alongside streams helps to filter sediments and pollution before they reach the stream, reducing the impact of erosion fromother parts of the watershed.